Abstract: Several damage accidents of nuclear power plant main transformers indicated that the accuracy and timeliness of the fault diagnosis of main transformer was poor. The status of nuclear power plant main transformer fault diagnosis technology were studied, the results show that the main transformer fault diagnosis workflow was too simple，and the information level of the main transformer fault on-line diagnosis system was low. The fault diagnosis method and preventive solution of power transformer were studied. A fault diagnosis workflow for the main transformer of nuclear power plant that incorporated mature power transformer fault diagnosis technology was designed. The optimization scheme based on the digital distributed control system to improve the function of the nuclear power plant main transformer fault on-line diagnosis system and the degree of system information level were given in this paper.